The present invention relates to a vat paper machine which comprises a cylinder-vat unit having a rotatable cylinder mold, a vat and a headbox connected to the vat for providing a continuous supply of stock to a predetermined level in the vat.
A drum filter of a similar type, in cooperation with an outer dewatering belt, is described in Norwegian Pat. No. 67086. The outer dewatering belt can be said to form a bottom in a vat and is formed of a plurality of strips arranged adjacent each other with slots for dewatering the stock. A first web is thus formed on the drum filter and a second web on the dewatering belt. The webs are then couched together between the dewatering belt and the drum filter to produce a unitary web. A drawback of this design is that irregular flows occur in the stock before the nip where the dewatering belt meets the drum filter since dewatering occurs at the dewatering belt right up to the nip. These flows have an unfavorable influence on the formation of the web in that parts of one or both webs are washed away so that the fiber distribution and grammage of the couched web become uneven. Such a drum filter cannot therefore be used for the production of security paper with true watermarks where considerable demands are placed on the formation and grammage of the web.
German published specification No. 2408304 describes a twin wire former for the production of security paper with watermarks. Stock is injected into the nip between a forming roll and an outer wire. This known apparatus is thus of a completely different type of former than that described herein.
Security paper with so-called true watermarks is generally produced on a cylinder-vat unit in which the wire cloth on the cylinder mold is provided with portions producing a different paper thickness. However, the speed of these formers is limited, as is their ability to produce paper of higher grammage.